1. Field of the Invention
The present invention relates to a manufacturing method for a semiconductor device and a semiconductor device manufactured according to the manufacturing method. The “semiconductor device” in this specification refers to an electrooptical device such as a liquid crystal display device or a light emitting device and an electronic device using them as a display portion.
2. Description of the Related Art
According to techniques widely employed in recent years, an amorphous semiconductor layer formed on an insulator, particularly, a glass substrate is crystallized, crystalline semiconductor layers are thus obtained, and thin-film transistors (which hereinbelow will be referred to as “TFTs”) are manufactured using the crystalline semiconductor layers as active layers. In addition, TFT electrical characteristics have rapidly been improved in recent years.
According to the recent technical advancement, signal-processing circuits of various types, which had been externally mounted using ICs or the like, initially, can be manufactured by using TFTs. Consequently, display devices in which a pixel portion and driver circuits therefor are formed integrally on the substrate have been realized. The displays using a reduced number of components are small and lightweight, and enable a significant manufacturing cost reduction to be implemented. As such, research and development in this field are widely advancing.
TFTs presently used are represented by amorphous silicon TFTs (each of which hereinbelow will be referred to as “a-Si TFT”) and polysilicon TFTs (each of which hereinbelow will be referred to as “p-Si TFT”). The a-Si TFTs are formed using the aforementioned amorphous semiconductor layer as an active layer, and the p-Si TFTs are formed using the aforementioned crystalline semiconductor layer as an active layer. Compared to the a-Si TFT, the p-Si TFT is superior in various aspects such as significantly high field-effect mobility. Thus, p-Si TFTs have high performance sufficient to form driver circuits of display devices of the type as described above.
However, since transistors used in IC chips or the like are formed on monocrystal silicon, the transistors have even higher electrical characteristics, and the electrical characteristics can be obtained uniformly. In comparison, the p-Si TFT has the semiconductor layer made from an aggregation of numerous crystal grains. While crystalline conditions are sufficient, respectively, electrical characteristics are significantly inferior because of, for example, variation in the orientation boundaries among the crystal grains (grain boundaries). Cases can occur in which a p-Si TFT is formed with an active layer containing a large number of grain boundaries, and variation occurs in the electrical characteristics because of variation in the number of grain boundaries or in the orientation of adjacent crystal grains. In other words, even in a case where TFTs of the same size are manufactured, and voltages of the same magnitude are applied to electrodes, respectively, variation still occurs in, for example, values of currents.
Operational amplifier circuits and differential amplifier circuits are given as representative circuits formed using transistors. These circuits include a current mirror circuit. As shown in FIG. 2A, the current mirror circuit is configured using two transistors 201 and 202, and is characterized in that a drain current I1 flowing through the transistor 201 is identical with a drain current I2 flowing through the transistor 202.
For example, an operational prerequisite condition of the current mirror circuit is that the transistors 201 and 202 are identical in characteristics. Even when the two transistors with variation in characteristics operate, since the condition of I1=I2 is not always ensured, the transistors do not function as an intended circuit. As such, ordinarily, transistors used to form a current mirror circuit are configured using identical materials in terms of, for example, the channel length and channel width. FIG. 2B is a diagram of an example layout of a practical current mirror circuit formed on a substrate.
FIG. 2C shows the configuration of a differential amplifier circuit using this current mirror circuit as an active load. In the circuit, when different potentials are applied to input terminals (In1 and In2), operation is performed satisfying the condition of I1=I2+I3 by utilizing the above current mirror circuit. In the operation, a potential difference between signals input to the input terminals In1 and In2 is amplified, and a waveform generated through the amplification can be obtained from an output terminal (Out) of the circuit. Also in this case, the circuit operates on the prerequisite condition that TFTs 211 to 214 are mutually identical in the electrical characteristics.
In practice, however, as long as the electrical characteristics vary in the p-Si TFT, even when the devices are arranged to have the same sizes, the variation cannot be suppressed. Consequently, the transistors are not suitable for manufacturing the circuit as described above.
Techniques for crystallizing an amorphous semiconductor layer include a technique in which a CW (continuous wave) laser is unidirectionally operated, and laser light is irradiated onto a semiconductor layer. According to this technique, crystal is grown continuously along the operation direction, and monocrystal is thus formed extending long in the operation direction. This technique is considered to enable crystal containing substantially no grain boundaries at least in the direction of the TFT. In this case, the crystal grains have a composition close to that of monocrystal, thereby being imparted with high electrical characteristics and uniformity.
Nevertheless, however, peeling-off can occur with a semiconductor layer deposited on a substrate during the irradiation of CW laser light thereto. When peeling of a semiconductor layer has occurred in a portion of the substrate, removal processing is performed on the semiconductor layer if possible to continue the manufacturing steps such that a semiconductor layer is re-formed. In this case, however, losses are inevitably involved due to the increase in the number of manufacturing steps. Moreover, according to a recent manufacturing method using a large substrate, since a large number of devices can be formed at a time on the substrate, even a loss of a single substrate results in a loss of a plurality of devices.